Sound waves are reflected whenever they experience changes in the medium where they propagating. Such changes can be for instance sudden temperature changes or simply a more or less rigid wall, which is place in the propagation path.
The original wave and the reflected wave will add up to a new field. Depending how both fields are related to each other different interference pattern will be created. In a three-dimensional space the pattern will be rather complicated and it will be difficult to visualise. However, sometimes you can hear the interference pattern, e.g. from a fan with very tonal character.          

In the animation one-dimensional wave propagation in the so-called Kundt's tube is presented. In acoustics we use the Kundt's tube for measuring the acoustic properties of material placed at the end of the tube. The interference pattern inside the tube is analyzed to obtain information about the reflection at the end of the tube. From the reflection one can conclude the acoustic properties of the material (i.e. the impedance of the material).

Three different boundary conditions are demonstrated; 
	rigid, 
	pressure release, 

	
	absorbing boundary with an absorption factor of  0.7

Keywords:

	standing wave

	propagating wave
	boundary conditions
	superposition of waves
